Despite careful medical exams and efforts to keep the players physically fit, it is very difficult for someone who weighs so much to be completely healthy, said Dr. Xavier Pi-Sunyer, an obesity expert at Columbia University in New York.

Herrion, 23, collapsed without warning Saturday and the 6-foot-3 (1.9 metre), 330-pound (150 kg) offensive lineman was pronounced dead at a Denver hospital. Officials are awaiting toxicology results after an autopsy failed to reveal the cause of Herrion's death.

"A sudden death like that in a 23-year-old with no evidence of a stroke would suggest that he had an arrhythmia," Pi-Sunyer said in a telephone interview.

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Certain athletes with high muscle mass can safely veer into BMIs of between 25 and 30, which would be considered overweight for the average person, but a BMI of 40 or higher cannot be considered anything but risky, experts say.

"The study uses BMI, which does not distinguish between fat and muscle," NFL spokesman Greg Aiello said today. "Any study that is done without taking into account body fat percentage is misleading."

Harp said it was unlikely the extra weight was due to muscle mass alone.

"The high prevalence of obesity in this group warrants further investigation to determine the short- and long-term health consequences of excessive weight in professional as well as amateur athletics," she wrote in her report, published in the Journal of the American Medical Association.

Pi-Sunyer agreed.

"When you get these huge lineman who are weighing 350 pounds ... then the chances are that they have more muscle but they also have significantly more fat," he said.

"We do know that excess fat brings with it certain risks. Included in these risks are primarily diabetes, high blood pressure and heart disease."

Also, he said, people with excess fat tend to have higher free fatty acids circulating in the blood.

"There has been some data to suggest that higher circulating free fatty acids could be a risk factor for increased cardiac arrhythmia," Pi-Sunyer said.

"He had just finished playing a vigorous game of football. Possibly, free fatty acid levels, given his weight and excess fatness, might have been elevated ever further after that."